Remove unused _TypeNone enum field. (flutter#14440)

* Remove unused field
* Similarly remove _TypeNone from web_ui

Author: aam

lib/ui/painting.dart

@@ -2632,8 +2632,6 @@ class ColorFilter {
   final int _type;
 
   // The type of SkColorFilter class to create for Skia.
-  // These constants must be kept in sync with ColorFilterType in paint.cc.
-  static const int _TypeNone = 0; // null
   static const int _TypeMode = 1; // MakeModeFilter
   static const int _TypeMatrix = 2; // MakeMatrixFilterRowMajor255
   static const int _TypeLinearToSrgbGamma = 3; // MakeLinearToSRGBGamma

lib/web_ui/lib/src/engine/color_filter.dart

@@ -114,8 +114,6 @@ class EngineColorFilter implements ui.ColorFilter {
   final int _type;
 
   // The type of SkColorFilter class to create for Skia.
-  // These constants must be kept in sync with ColorFilterType in paint.cc.
-  static const int _TypeNone = 0; // null
   static const int _TypeMode = 1; // MakeModeFilter
   static const int _TypeMatrix = 2; // MakeMatrixFilterRowMajor255
   static const int _TypeLinearToSrgbGamma = 3; // MakeLinearToSRGBGamma